1. Field of the Invention
The present invention relates to a method for forming an image which is obtained by employing an electrophotographic apparatus utilizing an electrophotographic process such as a copying machine, a printer and a facsimile.
2. Description of the Related Art
In an electrophotographic process, multiple methods such as a method disclosed in Japanese Patent Laid-Open No. 23910/1967 have been known. Ordinarily, a fixed image is formed through plural processes in which a latent image is electrically formed by various ways on a photosensitive material utilizing a photoconductive substance, this latent image is developed using a toner, the thus-developed toner latent image on the photosensitive material is transferred onto a transfer material such as paper and a sheet to reveal a toner image either via or not via an intermediate transfer material, then, the thus-transferred image is fixed on the transfer material such as the paper and the sheet The photosensitive material is optionally cleaned to remove the toner left thereon by various types of ways and the thus-removed toner is provided again in the plural processes described above.
As for a fusing technique for fusing the transferred image which has been transferred on a surface of the transfer material, a fusing method utilizing a heat roil system in which the transfer material (herein also optionally referred to as paper or sheet) on which the toner image is transferred is inserted into between a pair of rolls including a heating roll and a pressure roll to fuse the toner image. Such fusing method can obtain a firmly fused image at a high speed compared with other fusing methods whereupon energy efficiency is high and a detrimental effect to environment to be caused by vaporization of a solvent is small.
In recent years, an effort for increasing a copying speed has been paid in a field of the copying machines for the purpose of enhancing productivity, however, when fusing is executed at a higher speed by using a fusing apparatus of a nip roll system, it is necessary that same quantities of thermal energy and pressure as those in a case at a low speed are added to the toner and the paper. For this reason, it is necessary that a width of a pressure-contact portion (hereinafter referred to as xe2x80x9cnipxe2x80x9d) of a roll becomes larger in proportion with a process speed. As for a method of widening a nip width, there exist a method of increasing a load between the rolls, a method of increasing a thickness of an elastic body and a method of increasing a diameter of the roll. In the method of increasing the load or the method of increasing the thickness of the elastic body, a shape of a nip along a width direction may come to be uneven derived from a strain of the roll to cause a fusing unevenness or generation of a wrinkle on paper, hence, there intrinsically is a limitation on the load to be applied and a thickness of the elastic body. Further, in the method of increasing the diameter of the roll, though there is no such a quality problem of as described above, there is a problem that a size of the apparatus becomes large whereby a time (also referred to as warm-up time) for raising a temperature of the roll from a room temperature to a fixable temperature is prolonged.
In order to solve these problems and correspond to an increase of speed in operation, a method of using an endless belt (hereinafter referred to as xe2x80x9cbelt-nip systemxe2x80x9d) disclosed in Japanese Patent Laid-Open No. 132972/1986 has been propose. A fusing apparatus of the belt-nip system has an endless belt which is tightly stretched between plural support rolls in a rotatable manner, and a heat-fixing roll which forms the nip by contacting the endless belt Paper on which an undeveloped toner image is formed is allowed to pass through the nip formed by the heat-fixing roll and the endless belt to perform fixation by a pressure and a thermal energy generated in the nip. By taking such a constitution, a width of the nip to be formed by the endless belt and the heat-fixing roll can easily be larger than that formed by an ordinary nip-roll system whereby it becomes possible to correspond to the increase of speed in operation. Further, when compared at a same process speed, the constitution can be smaller in size than that of a fusing roll of the roll-nip system.
In the fusing method by means of the roll-nip system or the belt-nip system, the toner image and the roll or the belt arc contacted with each other whereupon, at the time of fusing, a phenomenon that a portion of the toner is stuck to the roll or the belt, that is, a so-called hot offset, is likely to occur. Particularly, when a fusing temperature of the fusing apparatus is high, an agglomerating force of the toner is deceased to cause a problem that the hot offset is likely to occur.
Further, in recent years, in order to decrease an energy consumption, it is desired that fusing can be performed at a lower temperature. Particularly, in order to attain a far higher degree of energy saving, it is desired to turn off electricity to the fusing apparatus when it is not in use whereupon it is necessary to raise a temperature of the fusing apparatus to a working temperature immediately after the apparatus is excited. For this reason, it is desirous to allow a heat capacity of the fixing apparatus to be as low as possible; however, on such an occasion, the temperature of the fusing apparatus tends to be fluctuated in a larger amplitude than usual. Namely, an overshoot of a temperature after the electricity is turned on becomes large and, at the same time, a decrease of a temperature derived from passing the paper becomes large. Further, a difference between temperatures of a portion through which the paper is passed and other portions to be derived when the paper having a width narrower than that of the fusing apparatus is continuously passed therethrough also become large. Particularly, in some case in which a high speed copying machine or printer is used, a power supply capacity is likely to be short whereupon such a tendency as described above is large. Therefore, the toner which can be fused at a low temperature and does not generate the hot offset up to a range of a far higher temperature, that is, has a wide fusing latitude is desired.
In order to decrease the fusing temperature of the toner, an adoption of a crystalline resin as a binder resin is proposed in Japanese Patent Laid-Opens No. 24702/1992, No. 24703/1992, and No. 329917/1997; however, though such a method as described above can decrease the fusing temperature, the hot offset resistance is not necessarily sufficient. Namely, though there is an effect of preventing generation of the hot offset to some extent by allowing a melted toner to be penetrated in paper, the melted toner tends to be excessively penetrated in the paper to cause a problem that an image having a uniform high density can not be obtained.
On the other hand, in order to prevent the generation of the hot offset, it is known that a low molecular weight polymer and a high molecular weight polymer are blended to produce a mixture which is then used as a binder resin of the toner containing a resin having an appropriate molecular weight distribution (disclosed in Japanese Patent Laid-Open No. 134652/1975) and it is also known that a resin having a crosslink therein is used (disclosed in Japanese Patent Laid-Open No. 23354/1975). However, these methods as described above can not obtain the wide fusing latitude enough to satisfy requirements of recent years. By using a large quantity of the high molecular weight polymer or a crosslinked polymer, it becomes difficult to generate the hot offset but, instead, the fusing temperature is increased. On the other hand, when a molecular weight of the low molecular weight polymer is decreased or a quantity of this polymer is increased in order to decrease the fusing temperature, a temperature of generating the hot offset is decreased. Further, the fusing temperature can be deceased either by decreasing a glass transition temperature (hereinafter also referred to as xe2x80x9cglass transition pointxe2x80x9d) or using a plasticizer, however, in this case, the toner is agglomerate-solidified in storage or in a developing apparatus, that is, a so-called blocking phenomenon easily occur.
For solving these problems, multiple techniques which do not use the crystalline polymer singly as a binder resin, but use it in combination with a non-crystalline polymer have been proposed as described below. To give some examples, it is disclosed in Japanese Patent Laid-Open No. 79860/1990 that the crystalline polymer and the non-crystalline polymer are simultaneously used; it is disclosed in Japanese Patent Laid Opens No. 163756/1989, No. 163757/1989, No. 81770/1992, No. 155351/1992, and No. 44032/1993 that the crystalline polymer and the non-crystalline polymer are chemically combined to produce a polymer. However, in a case in which a quantity of the non-crystalline polymer is larger than that of the crystalline polymer, the non-crystalline polymer becomes a continuous phase and the crystalline polymer becomes a dispersion phase. In this case, since the crystalline polymer is coated by the non-crystalline polymer, there is no problem to be caused by the crystalline polymer, on the other hand, since melting the entire tone is controlled by a softening temperature of the non-crystalline polymer, a low temperature fusing property can not be obtained.
Therefore, it means to request opposite characteristics that decrease of the fusing temperature and prevention of the hot offset are required at the same time; it is a present situation that the toner which can fully satisfy these characteristics at the same time has not been practically obtained.
Further, as a factor to be required for quality of a formed image, mentioned is gloss. It is preferable that the gloss can be controlled to be low when an image has a relatively low image density as a black-and-white image, while, though depending on a preference of a user, the gloss can be controlled to be in a relatively high range when the image has a high image density as a full-color image. Under these circumstances, in order to obtain the image having a high gloss, a polyester resin may be used as the binder resin and the toner containing the resin of a type having a larger storage elastic modulus GL than a loss elastic modulus GN may be used; however, on this occasion, the toner has a problem in hot offset resistance. On the other hand, in order to obtain the image having a low gloss, the toner containing the resin of a type having a relative high viscosity, and a larger loss elastic modulus GN than the storage elastic modulus GL may be used; however, on this occasion, the toner has a problem in the low temperature fusing property.
The present invention has been made in view of the above circumstances and provides a method of forming an image which solves the aforementioned problems present in the method of forming the image of the related art, is capable of forming the image at a high speed simultaneously, has a low temperature fusing property and hot offset resistance, and is capable of controlling gloss.
The present inventors have conducted intensive studies and found that a method of forming an image can be attained by selectively combining a specified fusing apparatus and a specified toner. The invention has been accomplished on the basis of this finding.
According to an aspect of the invention, the method of forming the image includes: a developing step of forming a toner image by developing an electrostatic latent image formed on a latent image holding member by a developer containing at least a toner, a transferring step of transferring the toner image formed on the latent image holding member onto a transfer material; and a fusing step of fusing the toner image on the transfer material. The toner contains at least a colorant and a binder resin in which a principal component is a crystalline resin, and the fusing step is performed by a fusing apparatus comprising a heat-fixing roll and an endless belt.
The fusing apparatus may include the heat-fixing roll which has a cylindrical core metal coated with a heat resistant elastic body layer and a heat resistant resin layer in this order and the endless belt having a pressure member inside, form a nip by winding the endless belt round the heat-fixing roll at a predetermined angle thereby allowing a recording sheet to pass through between the endless belt and the heat-fixing roll and generate strain on the heat resistant elastic body layer of the heat-fixing roll by urging the pressure member against the heat-fixing roll via the endless belt in the nip.
The heat resistant elastic body layer may be coated on the heat-fixing roll by a thickness of 0.5 mm or more.
The quantity xcex5 of the strain in a circumferential direction of the heat resistant elastic body layer may satisfy the following formula:
xcex5xe2x89xa70.5%
In the toner, all of a storage elastic modulus GL(90) and a loss elastic modulus GN(90) at an angular frequency of 1 rad/s and 90xc2x0 C. and the storage elastic modulus GL(30) and the loss elastic modulus GN(30) at an angular frequency of 1 rad/s and 120xc2x0 C. are 1xc3x97105 Pa or less, and a relation between the storage elastic modulus GL(90) and the storage elastic modulus GN(120) may satisfy the following formula (1):
logGL(90)xe2x88x92logGL(120) less than 2xe2x80x83xe2x80x83(1)
The melt viscosity of the toner at 120xc2x0 C. may be 100Paxc2x7S or more.
A melting point of the crystalline resin may be from approximately 50xc2x0 C. to approximately 120xc2x0 C.
The crystalline resin may be a crystalline polyester resin.
An ester concentration M defined by the following formula (2) of the crystalline polyester resin may be from approximately 0.01 to approximately 0.2:
M=K/Axe2x80x83xe2x80x83(2)
where M represents a ester concentration, K represents a number of ester groups in a polymer, and A represents a number of atoms constituting a polymer chain of the polymer.
Alternatively, the crystalline polyester resin may be an aliphatic polyester resin.
The toner may be prepared by emulsifying the binder resin and adjusting a toner diameter by agglomerating and fusing the emulsified binder resin.
The toner may add a release agent internally by from approximately 0.5% by mass to approximately 50% by mass.
The storage elastic modulus GL(30) of the toner may be 1xc3x97106 or more and the loss elastic modulus GN(30) of the toner may be 1xc3x97106 or more at an angular frequency of 1 rad/s and 30xc2x0 C.
The toner may have an area of temperature in which a change of a value of the storage elastic modulus GL and the loss elastic modulus GN is 1000 or more in a temperature range of 10xc2x0 C.
The toner may contain two or more external additives, and an average primary particle size of at least one type of the external additives may be in a range of from approximately 30 nm to approximately 200 nm.
The developer that forms the toner image may contain a carrier as well as the toner, the carrier having a nitrogen-containing resin coating.
The nitrogen-containing resin may be selected from the group of a urea resin, a urethane resin, a melamine resin, an amide resin and mixtures thereof.